The present invention relates to methods of producing artificial teeth.
As known in dental practice, in producing dentures, an outer crown is mounted on a primary part, for example an inner crown which is rigidly installed in tooth stump, by means of a releasable holding element. Frequently, so-called locking bars or in other cases friction pins are employed as holding elements. In the case of the utilization of locking bars the secondary part is held on the primary part by a form-locking connection, and in the case of the use of friction pins the secondary part is mounted on the primary part by friction via force-locking connection. In both instances it is necessary that bearing recesses should be produced so as to receive the holding elements in a right position and with exact fit.
If a friction pin is employed as a holding element it is usually fixed on the secondary part and is inserted into a groove-shaped recess formed in the primary part; in this case sufficient friction between the walls of the recess and the pin should be ensured to prevent the possibility of lifting of the secondary part from the primary part.
If rotary locking bars are employed as holding elements those bars should be by rotation easily, but without play, inserted into the respective bearing recesses. It is particularly important in the case of rotary or pushing locking bars that the respective holes or recesses formed in the primary part and in the secondary part should be exactly alighed with each other. When friction pins are utilized it is important that these pins should be rigidly mounted in the holes or recesses formed in the secondary parts so that the alignment or orientation of these pins will be defined by those holes. It should be noted that the quality of denture techniques depends upon the precision of shapes and alignment of the holes or grooves for receiving holding elements connecting the parts of the denture to each other.
The manufacturing of artificial teeth in the conventional art is performed in the following manner:
First, an inner crown, which is later rigidly installed in a tooth stump, is produced in a usual fashion. After usual casting, finishing and polishing of the inner crown, the outer crown is modelled from wax. In the metal inner crown one forms a groove for a locking bar by a milling cutter, after removing the outer crown from the wax model. Then the wax model is again installed on the inner crown and the groove will be exposed by means of a knife for wax modelling. An auxiliary wire pin is then inserted into the groove as a temporary post for a further locking bar, which wire pin is covered with wax. The wax model is thereafter again lifted from the inner crown and casted with the wire pin embedded therein.
In known methods, grooves for receiving holding elements in the inner crown are also milled or drilled whereas the bearing surfaces for the holding elements in the secondary part may be formed during the casting of the outer crown. In such a process required allowances can not be warranted, and the bearing grooves and surfaces in both parts of the artificial tooth must be precisely aligned later on. This in many instances requires time-consuming finishing work; for example drilling of one or more holes is required in order to obtain a desired fit of the outer crown on the inner crown on the one hand, and, on the other hand, to obtain proper slidable insertion of the locking bar into the respective hole without the use of excessive force.
Similar methods are used in manufacturing of bridges or partial dentures when, for example, a secondary part is anchored to a bar connecting two crowns or to a supporting frame of one crown. Bearing holes are then drilled or milled in the bar of the primary supporting frame whereas the bearing holes in the secondary part are produced by means of an auxiliary post during the casting process. Corresponding processing steps are used in the producing of so-called groove-shoulder pin-guides for sedimentary material, which are advantageously made for removable artificial teeth to provide proper guidance for sedimentary material always occurring on the oral surface. Therefore, guiding grooves are milled in the primary parts, which grooves also serve for increasing friction force and for guiding the outer or secondary parts to be aligned on the primary parts. The corresponding grooves in the secondary parts are produced during the casting.
Conventional methods are utilized when noble metal alloys are employed as a working material for inner and outer crowns or primary and secondary parts. When non-noble metal alloys are employed such known methods can not be used because due to the great hardness of those alloys, for example in case of Cr-Ni-alloy, the drilling of holes in such materials by known methods is impossible. Due to the fact that in working with non-noble metal alloys substantially high temperatures are involved, grooves for receiving holding elements can not be produced by casting either, because usual auxiliary posts are not suitable for such temperatures. The customarily used soldering of the connecting elements in the parts of the denture with non-noble metal alloys is impossible since it causes subsequent absorption of the solder by ceramic facing material. Finally, it should be considered that in a casting process with non-noble metal alloys, the reaction between the flowing metal and an embedded mass takes place, which can cause the formation of a thin layer on the surface of the metal. The thickness of the layer depends upon a number of factors and can not be anticipated. Therefore, small deviations in the size of cast objects are unavoidable so that sufficient friction of two telescopically installed elements, with the outer part of the non-noble metal alloy, can not be warranted. In brief, it should be noted that good-quality artificial teeth from non-noble metal alloys could not be produced by conventional methods although it is has been desired to use such alloys in dental techniques since costs of noble metals constantly increase lately.